I thought I had planned my garden quite carefully, but having been on the Introduction to Forest Gardening course at ART recently I realised that I had not worked out a nitrogen budget. If the garden is to be truly sustainable it is important that it doesn't rely on imported nutrients. I've also been thinking about adding nitrogen fixing plants into the design, but want to know how many I need.

The principles for the budget are simple: 1. estimate the nitrogen requirements for the plants in your garden2. estimate the nitrogen inputs3. see if the inputs meet the requirements and if not increase the sustainable inputs through including more nitrogen fixers or other sources.

I used Martin Crawford's Creating a Forest Garden as a guide. It has figures for annual nitrogen requirements for different classes of plants, in order of decreasing requirements: annual vegetables, heavy cropping (apples, pears etc.), moderate cropping (e.g. currants) and undemanding (e.g. hawthorn) that don't need additional feeding. I calculated that when the garden is mature I will have about:

Annual vegetables: 8m2

Heavy cropping plants: trees 92m2, shrubs 12m2, total 104m2

Moderate cropping plants: trees 26m2, shrubs 15m2, total 41m2

Doing the sums gives a total nitrogen requirement of 1.139kg of nitrogen each year. So where is it all going to come from without resorting to importing material from outside in an unsustainable way? The key sources are nitrogen fixing plants, compost and human urine. The calculations for my plot at the moment are as follows:

Nitrogen fixing plants: Not many at the moment. I have crimson clover sown through much of the lawn. I have plants which accumulate nitrogen (comfrey and nettles), but as I understand it these don't add nitrogen to the system only concentrate it from the soil and subsoil and can be cut and applied as a mulch or to compost. Based on the number of comfrey plants and how often I cut them this accounts for only 0.006kg per year.

Compost: I've got a couple of compost heaps. Now that most of the main planting has been done in the garden I use this for top dressing to feed plants and to improve the thin soil. I estimated that I empty the kitchen compost bin every three days. It's raw contents normally weight about 1.7kg. Creating a Forest Garden suggests compost has about 0.5g/kg. I found a reference somewhere on the web that suggested that raw compost reduces in mass by about 10-30% when it is composted. I assumed mine reduces in mass by 20%, giving about 165kg of compost and 0.825kg of nitrogen each year. That is getting close to the requirement value.

Human urine: This is something that makes some people uncomfortable, but it is a fantastic source of nitrogen in a form that is readily available to plants. I first read about it's use as a compost accelerator and learnt from experience that it works. I've used it to turn a pile of chipped Leylandii into compost in just a few months. A quick search of the web gives lots of information on its potential use in gardening and agriculture and of course it saves water used in flushing it away. There is even a great little book that makes the case as well as recent scientific studies. It can even help to keep cats and other animals away. So doing the sums - I estimate two pees a week from myself and my male offspring (counted as half an adult), for six months of the year. The total is 0.576kg per year during the growing season.

In total that makes just over 1.4kg as inputs. That is more than enough for the mature garden requirements calculated above. So it looks as though in a small home garden of this size compost and urine can meet all the nitrogen needs. I think this is one area where a small home forest garden will differ from a larger plot. In a back garden situation there is likely to be more compost from kitchen waste and more human urine available for a given area than in a larger plot where nitrogen fixers will have to play a more important role. The pressure for space for cropping plants is also heavier in a back garden. However, I am still planning to include some nitrogen fixing plants, particularly in the front garden where there is more sun and urine is not as easy to apply!

So far I've only completed a nitrogen budget. I should probably do a potassium one next. The main difference will be that wood ash will contribute to that budget. Maybe that will be a future post.